The present invention relates to a bath reboiler-condenser, condenser, of the type comprising at least one exchanger body which comprises a stack of parallel plates, closure bars and spacer-waves which define a series of vaporization passages which are entirely open at their top and bottom ends, and a series of condensation passages.
The invention applies in particular to the main reboiler-condensers of air distillation plant which vaporize liquid oxygen at low pressure (typically a pressure slightly greater than atmospheric pressure) by condensing nitrogen at a medium pressure (typically an absolute pressure of 5 to 6 bars), and it will be explained hereinafter in that application.
Bath reboiler-condensers operate as a thermosiphon. The upward flow of the vaporizing oxygen is provided by the hydrostatic pressure due to the head of the liquid oxygen bath and to the reduction in weight of the vaporizing liquid.
For safety reasons, the recirculating liquid oxygen flow rate must be several times greater than the vaporized oxygen flow rate. For this reason, the head of the liquid oxygen bath must be approximately equal to the height of the exchanger, i.e. the exchanger is almost totally immersed in the liquid.
To reduce the temperature difference between the fluid which is condensing and the fluid which is vaporizing, in order to reduce the pressure of the heating nitrogen and therefore the energy required to compress the treated air, the heat-exchange area must be increased. Because the horizontal dimensions of the exchanger are limited by the space available in the bottom of the low-pressure distillation column, the height of the exchanger must be increased.
However, increasing its height increases the hydrostatic pressure of the liquid at the bottom of the column, i.e. at the inlet of the vaporization passages. This creates in the lower part of the exchanger a region containing undercooled free liquid and at least partially neutralizes the beneficial effect of increasing the heat exchange area.
The object of the invention is to improve the efficiency of the reboiler-condenser with a relatively simplified construction.
To this end, the invention provides a reboiler-condenser of the aforementioned type characterized in that the vaporization passages are subdivided by separator bars into at least two stacked vaporization regions each of which is entirely open at its bottom end and at its top end, each vaporization region being provided with a liquid recirculation corridor and overflow means at the top whereby the liquid overflows into an underlying vaporization region to create in each vaporization region a partial-height bath separated from the other bath(s) and in which said region is immersed to substantially all of its height, in that the top end of each bottom vaporization region and intermediate vaporization region is spaced vertically from the separator bar which supports the next higher bath, and in that the heat-exchanger body includes means for supplying liquid to the top vaporization region and means for evacuating vapor at each intermediate level between two vaporization regions.
The invention also provides air distillation plant including a main reboiler-condenser as defined hereinabove for vaporizing liquid oxygen by condensing nitrogen.